1. Field of the Invention
This invention relates to the use of membranes for the recovery of nitrogen from air. More particularly, it relates to the operation of such membranes under variable temperature conditions.
2. Description of the Prior Art Recent developments relating to permeable membranes have served to significantly reduce the cost of on-site systems for the production of relatively low purity, small tonnage nitrogen. The inherent simplicity of permeable membrane systems provides a strong incentive for the development of such systems to satisfy the requirements of a wide variety of commercial operations. The availability of hollow fiber membranes and membrane modules has enhanced the development of simple processes and systems for the production of product nitrogen and oxygen from air.
In the use of membranes for air separation, feed air is compressed and passed along the outside (inside) of a hollow fiber membrane bundle, with oxygen preferentially permeating through the surface of the membrane and with nitrogen being preferentially retained as a non-permeate or retentate stream on the feed side of the membrane. As it progresses through the membrane, the retentate stream becomes richer in nitrogen, so that the retentate is withdrawn from the discharge end of the module, on the feed side thereof, as a nitrogen-rich product.
The efficiency of membrane processes for air separation depends on the properties of the membrane material employed and on the parameters of the operating process. Two membrane material properties are of particular significance, namely the permeability/thickness ratio for oxygen, P.sub.o /t, and the selectivity, or separation factor, .alpha., which is the ratio of the permeability of oxygen to that of nitrogen. The membrane process efficiency is enhanced when both of these factors is increased. Both factors are temperature-dependent. In general, the value of (P.sub.o /t) increases with increasing temperature, while the value of .alpha. decreases as the temperature is increased. For fixed values of pressure and other operating variables, there is thus an optimum operating temperature. The range of properties available from many early hollow-fiber membrane materials was such that the optimum operating temperature was above ambient temperature. It is relatively easy to attain and conduct membrane air separation operations at this above ambient temperature, as by utilizing some or all of the heat of compression of the feed air or by modest heating of the feed stream. For consistent operation, membrane materials have been designed to operate typically at a fixed feed temperature generally from about 90.degree. F. to about 140.degree. F., and independent of the ambient temperature conditions. Less sophisticated membrane systems and processes operate with no feed temperature control. In this case, the operating temperature is determined solely by the existing temperature conditions and such systems and process are commonly limited to indoor use.
Improvements in materials and in the manufacture of hollow-fiber membranes have led to the development of advanced membrane materials, in addition to materials previously known, having high inherent values of P.sub.o /t, with optimum operating temperatures that are below ambient temperature. To accommodate this circumstance, the feed temperature can be reduced somewhat by the use of compressor aftercoolers. When temperature reduction by this means has been exhausted, more expensive means of refrigeration are required to maintain the membrane feed stream at the low optimum operating temperature, particularly when the ambient temperature is high. The use of external means of refrigeration will be understood, however, to erode some or all of the advantages of operating at the optimum low temperature of the membrane system. As a result, membrane process technology typically does not take advantage of the lower temperatures that may be optimum for improved operation of advanced hollow-fiber membranes. Most membrane plants continue to operate with feed gas temperatures well above the ambient temperature and accept the inefficiencies resulting from such operation above the low optimum operating temperature for the advanced membrane materials employed.
There is a genuine need in the art for further developments to enable newer, advanced membrane materials having higher inherent P.sub.o /t values and optimum temperatures below ambient to be operated effectively without the need for external refrigeration. Such developments need to provide an economically feasible means for utilizing the advanced membranes in practical, commercial operations over the course of varying ambient temperature conditions.
It is an object of the invention, therefore, to provide a process for air separation that enables the properties of advanced, higher permeability membranes having below ambient optimum operating temperatures to be effectively utilized under varying ambient temperature conditions.
It is another object of the invention to provide an improved process for the use of advanced, high permeability membranes without the use of external refrigeration to achieve the low optimum operating temperatures thereof in times of high ambient temperature conditions.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.